On the left, a 2-Liter bottle with a high pressure Oz Top. On the right, a 3 liter glass bottle with a rubber bung and bubble air lock. This air lock contains sulfite water, but I've hear of people having very good luck using vodka as a germ-bacteriacide in their airlocks.

Using a bubble trap allows you to count the Bubbles Per Minute (BPM) to help figure out when fermentation is just about done.